Corrosion resistant ultra high strength stainless steel

ABSTRACT

Corrosion resistant stainless steels capable of developing very high tensile strengths, e.g., upwards of 350,000 psi, in the work hardened condition, the steels containing correlated amounts of chromium, molybdenum, nickel, cobalt, carbon, nitrogen, manganese and silicon. The steels are specifically directed to minimizing corrosive attack in low velocity seawater.

Unlted States Patent 1 1 1 1 3,772,905 deBarbadillo, II Nov. 13, 1973 [54] CORROSION RESISTANT ULTRA HIGH 3,1319]; 12/1333 llZayszn zvv ran S t STRENGTH STAINLESS STEEL 2,294,803 9/1942 Rich 75/l28 W [75] Inventor: John Joseph deBarbadillo, 1!, 2,306,662 12/1942 Krivobok 75/128 W Suffern, N.Y. 2,398,702 4/1946 Fleischmann... 75/128 W 2,801,916 8/1957 Harris 75/128 W [731 Asslgnee g z z g gf CmnPany, 3,152,934 10/1964 Lula 75/128 w ew or 1 3,563,729 2/1971 Kovach 75/128 W Oct 13, Denhard W PP 80,341 Primary Examiner-Hyland Bizot Att0rneyMaurice L. Final [52] U.S. Cl. 75/126 C, 75/126 H, 75/128 B, ABSTRACT 75/128 W [51] Int. Cl. C22c 39/14, C22c 39/20 rr n r istan stainless steels capable of de elop- [58] Field of Search 75/128 W, 128 B, g y g tensile r g go upwards f 75/126 C, 126 H; 78/128 B 350,000 psi, in the work hardened condition, the steels containing correlated amounts of chromium, [56} References Cited molybdenum, nickel, cobalt, carbon, nitrogen, manga- UNITED STATES PATENTS nese and silicon. The steels are specifically directed to 2,083,524 1/1937 payson I I 75/l28 W minimizing corrosive attack in low velocity seawater. 2,115,733 5/1938 Krivobok 75/128 B 22 Claims, 2 Drawing Figures 2 WM 26 l 25 F a a 2 ,7 24 \\4V, h Q 5 Q 25 G 7 I I D L5 14 A llF/F/IE' fivaax CORROSION RESISTANT ULTRA HIGH STRENGTH STAINLESS STEEL The subject invention is addressed primarily, though not exclusively, to the problem of producing marine stranded wire characterized by outstanding resistance to crevice and pitting corrosion attack in quiet seawater and possessing ultra high tensile strength, e.g., upwards of 350,000 to 400,000 psi, together with adequate ductility, characteristics particularly useful in deep ocean mooring and working cables.

Not long ago, it was reported that the estimated loss occasioned by the ravages of corrosion approximate five billion dollars annually. Not least among the contributors to this dismal picture have been the aqueous chloride solutions, notably seawater and its environs. And the extensive technological efforts currently being expended in such fields as oceanography, desalination and offshore drilling, etc., presage an even greater loss unless more corrosion resistant materials are developed.

As those skilled in the art are aware, certain stainless steels, among other materials, by virtue of their well established corrosion resistant qualities, have been used to some extent in combating seawater attack. For example, AISI 316 was developed to inhibit crevice and pitting corrosion in seawater as well as other environments. However, in common with virtually all conventional austenitic stainless steels, A181 316 has been particularly prone to attack under stagnant or low velocity seawater conditions.

By way of explanation, the corrosion resistant nature of the stainless steels, as is now generally accepted, is attributable mainly to an ability to assume what is commonly referred to as the passive state. This is an inherent phenomenon due principally to composition and involves the formation of a thin, continuous, rather tenacious film (mostly chromium oxides) about the surface, the film acting as a barrier to penetration by corrodents. However, the passive film is all too frequently ruptured. This usually occurs at localized areas and the ruptured sites tend to form crevices or pits, conditions most conducive to attack. These flaws or imperfections, may be (a) unintentionally induced, e.g., nicks, scratches, dents and the like, or (b) formed as the result of natural processes, e.g., through the adherence of barnacles or other marine organisms, or (c) inescapably brought about, stranded cable, valve seats, etc., being illustrative. Moreover, the chlorides are notorious for their ability to materially assist in the breakdown of passive film.

The problem is particularly serious in low velocity or stagnant seawater, notwithstanding an innate ability for such passive films to undergo self-healing at the point of rupture. This is to say, under moderate or high velocity conditions, there is sufficient oxygen in seawater brought to the point of rupture to enable new chromium oxides to form and thereby reestablish the film. But, although other considerations are undoubtedly involved, under stagnant or low velocity conditions, e.g., less than about 3 or 5 feet per second (fps), the amount of available oxygen in seawater is generally insufficient to permit the necessary replenishment of oxides. This prevents or inhibits the film from reforming. And again, chlorides interfere with the natural processes which promote film restoration.

Complicating the problem herein is the concomitant desideratum for exceptionally high tensile strength coupled with adequate ductility, properties which in and of themselves hardly go hand-in-hand. Apart from corrosion resistance considerations, standard austenitic stainless steels, generally speaking, can be worked to strength levels circa 300,000 psi. But this is not high enough at least where fabricated end products need to manifest strength levels of, at least, 325,000 or 350,000 and upwards of 400,000 psi. Too, the very effort to achieve high tensile strength focuses attention on ductility at the given level of high strength and also on certain processing characteristics. For example, while some of the ferritic or martensitic stainless steels might offer enhanced resistance to seawater attack, they often are not only difficultly workable but in the production of various products they generally lack sufficient ductility at the given high strength level to be formed or fabricated into certain desired forms, e.g., twisting wire into stranded cable.

It has now been discovered that the type of corrosion resistance, tensile strength and ductility above described can be brought together in one stainless steel provided the steel contains correlated percentages of chromium, molybdenum, cobalt, nickel, carbon, etc., as described herein.

It is an object of the invention to provide stainless steels novel in composition.

It is a further object to provide stainless steels highly resistant to low velocity seawater attack and which are capable of exhibiting extremely high levels of strength and adequate ductility when work hardened into various product forms such as wire.

A further object of the invention is to provide strong, corrosion resistant stranded cable and other articles such as fasteners, reinforcing members, springs, cold forgings, etc.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a graphical'representation correlating the herein described Ferrite Index and Austenite Index; and

FIG. 2 is also a graphical representation correlating the herein described Ferrite Index and Austenite Sta bility Index.

Generally speaking, the subject invention contemplates stainless steels containing (in weight per cent) from about 14 to 27.65 percent chromium, from 3 to 12 percent molybdenum, at least one of, and most advantageously both, nickel and cobalt in amounts up to 17.5 percent nickel and up to about 25.5 percent cobalt, carbon in a small but effective amount, e.g., 0.01 percent, up to 0.3 percent, up to 0.3 percent nitrogen, the sum of the carbon plus nitrogen not exceeding 0.3 percent, up to 2 percent silicon, up to 5 percent manganese with the balance being essentially iron, the steels being characterized in the annealed condition by a predominantly austenitic microstructure, the austenite, however, being of such instability that a relatively high rate of work hardening is achieved through progressive strain-induced transformation of at least a portion of the austenite to martensite.

In addition to the requirement that the compositions formulated satisfy the foregoing ranges, it is indispensable in achieving the necessary combination of strength, ductility and corrosion resistance that the Corrosion Indicator comprised of chromium plus twice the molybdenum be greater than 28.5 and that the percentages of the alloying constituents be otherwise controlled so as to give correlated Ferrite Index, Austenite Index and Austenite Stability Index values with regard to the diagrams depicted in FIGS. 1 and 2. In this connection, the Ferrite Index is given by the relationship %Cr %Mo 1.5 (%Si) about 22 to 30.65,

the Austenite Index by the relationship %Ni 0.65 (%Co) %Mn l(%X) 20(%Y) 14.1 to 29.9

and the Austenite Stability Index by the relationship %Ni 0.15 (%Co) %Mn +(%X)+ 20(%Y) 4.25 to 17.4

(In equations (12) and (0) above, X represents the total carbon plus nitrogen in solution up to a value of 0.1 percent and Y represents the total of these elements in solution above 0.1 and up to 0.3 percent.) Accordingly, a given value for the Ferrite Index must be correlated with the Austenite Index and Austenite Stability Index so as to respectively represent a point within the area ABCA of FIG. 1 and the area JKLMJ of FIG. 2 of the accompanying drawing.

While it is difiicult to assess the individual effects of each of the essential constituents owing to their interdependency, it is decidedly beneficial in consistently achieving a high level of corrosion resistance that the Corrosion Indicator be at least 30 or 31. While this value can be lowered to near 28.5, resistance to aggressive corrosive environments is reduced.

Too, though the sum of the respective percentages of chromium plus molybdenum can be as high as 30.65 percent, it is much preferred that this total sum not exceed about 27.5 or 28 percent. The primary reason for this stems from the need to minimize the occurrence of delta ferrite, a phase which promotes hot working difficulties (e.g., decomposition of ferrite to brittle intermetallic compounds) and which, among other things, tends to result in a lower rate of work hardening by reducing the volume of austenite available for transformation to martensite. This can result in lower strength. Up to 5 or 6 percent delta ferrite can be tolerated but it is advantageous that it not exceed 2 or 3 percent by volume.

The risk of delta ferrite formation, is in accordance herewith, greatly minimized through a carefully balanced alloy chemistry. This might be best explained with reference to FIG. 1. Alloys in which the correlated value for the Ferrite and Austenite Indices fall to the left and above the line DE contain virtually no delta ferrite upon cooling from preferred annealing temperatures, e.g., about 2100F. and up to 2200F. Should this value fall below and to the right of line DE, the appearance of delta ferrite can be expected in amounts up to 5 or 6 percent, particularly with annealing temperatures of, say, 2150F. and above, the amount increasing as the distance from line DE towards AB increases. Nickel exerts a most positive influence in inhibiting delta ferrite. For this and other reasons, the steels preferably contain at least 1 or 2 to 10 percent nickel.

It might also be added that compositions within area DEF are deemed outstanding, particularly with a combined chromium plus molybdenum content of at least 24 percent since, in addition to being devoid of detrimental amounts of delta ferrite, they exhibit a high rate of work hardening at room temperature. This precludes or greatly minimizes any need for working at below room temperature, say, down to l00F., as would be required for compositions falling between line F H and line CB of FIG. 1, the temperature decreasing as CB is approached. Alloys within the area DEI-IG not only give excellent results but for the most part are the least costly.

As indicated above, the high tensile strengths characteristic of the steels is brought about by a combination of work hardening of the austenite accompanied by strain-induced transformation of at least a portion of the austenite to martensite. To achieve the necessary strength levels, the steels must be austenitic as annealed. But, even should a steel be completely austenitic upon cooling from annealing temperature, if it is too stable, the desired level of strength will not be achieved. The proper balance is brought about through correlation of the above described Indices, particularly the Ferrite and Austenite Stability lndices as reflected by FIG. 2. At this point, it might be pointed out that 00- balt plays a significant role in that while it promotes the formation of austenite during annealing, it only mildly depresses the M temperature. If it were to depress the M, temperature to the degree of, say, nickel or chromium, then the alloy might be too stable to be work hardened such that a strain-induced transformation could readily by achieved. At best, unnecessary low work hardening temperatures and attendant problems would be likely. A cobalt range of from 15 to 25 percent is distinctly beneficial.

In any case, alloys below and to the left of line JK work harden too rapidly to be of commercial interest, whereas compositions above ML are too stable. From RS to ML work hardening should be carried out at below about room temperature. Steels exhibiting increasing strength are obtained with compositions away from RS and toward .IK. Within the area defined by PQRS, excellent strengths can be achieved and wire in the fully worked condition exhibits sufficient ductility for the Wrap" and Kink tests described herein. As the area contiguous to .II( is approached from PQ, the rate of work hardening increases and some difficulty may be experienced in respect of the higher percentages of reduction accomplished through working.

It is preferred that the silicon and manganese contents not exceed about 1 and 2 percent, respectively. Silicon is a strong ferrite stabilizer and as the amount thereof is increased where is required either the presence of higher nickel and/or cobalt contents (thus adding to cost) or the use of lesser amounts of chromium and/or molybdenum, (thus in a given case likely detracting from corrosion resistance). Manganese tends to promote the formation of sigma and this can be harmful.

Although the total sum of carbon plus nitrogen can be as high as 0.3 percent, it is of considerable advantage that this total not exceed 0.1 to 0.2 percent and that carbon be present in an amount of at least 0.01 or 0.02 percent and up to 0.1 or 0.15 percent, e.g., 0.01

to 0.08 percent, with the nitrogen not exceeding 0.1 percent, e.g., not exceeding about 0.04 or 0.05 percent. These elements contribute significantly to the strength of drawn wire or other work-hardened products but ground to a thickness of 0.225 inch and cold rolled to 0.050 inch. One inch square specimens were cut therefrom, deburred and ground to a 240-grit finish.

To simulate expected corrosion behavior in stagnant also markedly depress M, temperature, particularly 5 seawater, an accelerated test was adopted. This test,

when the total content exceeds about 0.1 percent. This described in the Journal of the Electrochemical Sociin turn, can result in a steel more stable in the annealed ety, Vol. 103, pps. 375-390, No. 7, July 1956, is concondition than is necessary. sidered comparable to long time exposure in seawater.

in carrying the invention into practice, commercially Specimens were wrapped with about six loops of a rubpure as well as high purity materials can be employed. 10 ber band about their surface to form intentional crev- The steels can be produced in accordance with convenices, and then immersed in an aggressive corrosive tional practice as those skilled in the art will readily apcomprised of a 10 percent ferric chloride solution for preciate. However, it is preferred that soaking temperaa period of ab ut 72 hours, the solution being maintures not exceed about 2250F. to 2300F. lest the fortained at about room temperature. The specimens were mation of excessive delta ferrite be promoted. Annealwithdrawn, rinsed and the corrosion products removed, ing treatments are beneficially conducted over the the specimens being then dried and Weighed.

range of 2100F. to 2200F. In this regard, in applica-. At least two specimens of each composition were so tions requiring severe reductions in area such as is the tested and the results, also reported in Table I, p

cas f d i i t i h use of an i di s ent the average value. The weight loss was used as the thermal treatment an be d id dl b fi i l i many indicia of resistance to corrosion with the following instances in achieving the highest level of strength toevaluation being madei A weight 1055 of not more than gether with adequate ductility. This will be demon- 0.5 milligram indicates virtual immunity, there being strated hereinafter but it i th f note to point out no pitting or visible marks associated with the crevices.

that as a on e f proper th l treatment l Slight crevice corrosion reflects a weight loss of from costly alloyed steels can be used. about 0.5 to 5 or even 10 milligrams, there being no pit- For the purpose of giving those skilled in the art a ting but some slight discoloration at at least one edge better understanding of the invention, the following deof contact with at least on loop of the rubber band.

scription and illustrative data are given. Moderate to severe crevice denotes a weight loss above To conduct corrosion tests a number of compositions l0 an p I0 100 milligrams, there being p g but within the invention, Table 1, were prepared using a a greater degree of discoloration plus grooving. To be vacuum induction furnace (except Alloy 6 which was within the present invention, the weight loss should not melted in argon and poured in air). The materials embe greater than about 20 milligrams for the test deployed were of high purity, e.g., the electrolytic forms scribed. Advantageously, the weight loss should not exof iron, nickel and cobalt being used together with moceed about 10 milligrams. Alloys A and B, composilybdenum pellets, Shieldalloy chromium, low carbon tions outside the scope of the invention, are included ferromanganese, spectographic grade carbon etc. l n i for purposes of comparison. 7

TABLE I Cr Mo Ni Co C Si Mn Wt. loss.

(percent) (percent) (percent) (percent) (percent) (percent) (percent) Mg Note: The balance of the alloys was iron plus impurities.

tial or partial deoxidation was accomplished through the addition of carbon to the charge and ferromanganese was added with the chromium. Final deoxidation comprised the use of aluminum, zirconium and calcium-silicon. The alloys were poured at approximately 2750F. and ingots (30 1b., 3% inch squares) were soaked for two hours at 2200F., forged to 3 inch sections, reheated at 2200F., forged to 2 inch sections, cut in half, reheated to 2200F., and then rolled to 0.585 inch RCS bar and V4 inch plate.

Specimens of the A inch plates were annealed at 2200F. for one hour and water quenched, surface subjected to intermediate thermal treatments (Table III). With regard to the former, the b inch bars were machined and first drawn to 0.103 inch or 0.062 inch diameter on a draw bench using copper plate and soap lubricant, the copper plate being removed and the specimen annealed where necessary. Subsequent to sand blasting and pickling after annealing, the wires were drawn down to final desired diameter using diamond dies and oil lubricant. Three specimens of each composition were drawn, usually a reduction in area of 90, 98 and 99 percent being performed. The Kink test comprised making a loop of wire and drawing tight by hand such that virtually no light appeared through the kink. The Wrap test consisted of forming successive contiguous turns of the wire about its own 15 boosted by thermal treatment as will be seen from a perusal of Table 11]. As to Alloys 20 and 21, these failed to pass both the Kink and Wrap tests and thus their use for stranded cable would be unsuitable although they would be useful for fasteners, particularly coldthreaded fasteners, and other cold worked products such as sheet, reinforcing bar, etc.

Turning to the results set forth in Table 111, Heat Treatment A" comprised reducing the rod to a 51 mil diameter and then heat treating at 800F. for 15 minutes and thereafter drawing to desired diameter. For Heat Treatment 8" a temperature of 1000F. was used whereas in Heat Treatment C" a temperature of 1200F. was employed. Heat Treatment B was, in effect, an aging treatment with Heat Treatment C diameter. being an austenitic reversion treatment, i.e., the mar- TABLE 11 Cr Mo Co Ni C Si Mn Red LTS Kink Wrap Alloy No {percentl (percent! (percent) (percent) (percent) (percent) (percent) (percent) ksi test test 98 375 P P 99 385 P P 98 404 M P 99 413 M P 17 18.0 6.0 15.6 6.2 .085 .30 .17 90 290 98 345 P P 99 346 P P 18 18.2 6.0 13.9 7 4 .094 .30 17 90 282 98 314 P P 99 321 P P 19 17.1 5.7 11.3 14.7 .050 .30 .17 90 263 98 321 P P 99 320 P P 20 18.1 6.2 21.5 2 5 .079 .30 .10 345 98 413 F F 99 423 F P 21 17.6 6.1 24.3 n.a .099 .30 .18 90 346 420 F F 97 408 F P P Passed. M Passed but marginal. F= Failed. UTS= Ultimate tensile strength. Red Percent drawn.

Concerning the data given in Table II, Alloys 15, 16 4Q tensite formed during drawing to a 51 mil diameter was and 17 all reflect that exceptionally high tensile strengths can be achieved in wire form together with adequate ductility. Alloys l8 and 19 are marginal in respect of strength, the level of which was significantly virtually completely transformed back into austenite. This austenite was quite unstable and readily retransformed into martensite upon further drawing to final diameter.

TABLE 111 Heat treatment Ductility H H a Alloy No. R.A. U.T.S. U.T.S. U.T.S. U.T.S. Kink Wrap Kink Wrap Kink Wrap 90 314 365 377 320 15 98 375 410 434 412 P P M P P P 99 785 425 448 419 P P F P P P 90 337 404 brittle 377 l6 98 404 452 brittle 449 F F F P 99 413 465 brittle 455 F P F F 90 282 327 316 313 17 98 314 378 350 363 P P P P P P 99 321 363 353 376 P P P P P P 90 282 301 309 300 l8 98 314 358 312 328 P P P P P P 99 321 342 317 333 P P P P P P 90 263 295 293 288 l9 98 321 336 326 326 P P P P P P 99 320 336 325 333 P P P P P P 90 345 452 brittle 404 20 98 413 410 brittle 463 F F 99 423 474 brittle F F F M 90 346 422 brittle 437 F F F P 21 95 420 462 brittle 1 482 F F F F 97 408 brittle 96% R.A. 7 93% RA.

Thermal treatments A and C readily reflect that substantial increase in strength can be obtained. The data further reflect that an aging treatment, Heat Treatment B, should be avoided because of the tendency toward severe embrittlement. Where an intermediate thermal treatment is used, the temperature should be from about 500F. or 700F. but should not exceed about 900F. or possibly 950F., unless the structure of the steel is first transformed back to austenite by reversion as illustrated by Treatment C. If an austenite reversion treatment is used, it should be followed by a significant amount of subsequent cold reduction to minimize possible loss in corrosion resistance. The period for intermediate thermal treatment can be extremely short, e.g., 1 minute.

It should be further pointed out that the overall effectiveness of the thermal treatment is dependent upon martensite content of the work hardened product. No significant increase in strength is derived with fully austenitic steels, i.e., those containing less than about percent martensite.

On the other hand, wires which are fully martensitic tend to severely embrittle. At the time of employing an intermediate thermal treatment, the work hardened product should contain at least about 10 percent to not more than about 60 percent martensite.

In accordance with the invention, steels having the following compositions have been found to be particularly advantageous from the overall viewpoint of corrosion resistance, strength and ductility: from about 16 to 22 percent chromium, about 4 to 8.5 percent molybdenum, the Corrosion Indicator being at least 30, from 2.5 to 6.5 percent nickel about to 25 percent cobalt, about 0.01 to 0.15 percent carbon, up to 0.1 percent nitrogen, the sum of the carbon plus nitrogen not exceeding about 0.2 percent, up to .75 percent silicon, up to about 2 percent manganese and the balance essentially iron. Another compositional range which gives outstanding results has from 17 to 20 percent chromium, 5.5 to 7.5 percent molybdenum, the Corrosion Indicator being about 30.5 or more, about 3 to 5 percent nickel, about 17 percent to 23 percent cobalt, 0.01 to 0.08 percent carbon, up to 0.04 percent nitrogen, up to 0.75 percent silicon, up to l or 2 percent manganese, with iron being essentially the balance. In each instance, the constituents must be formulated such that a correlation of the Ferrite, Austenite and Austenite Stability Indices is represented by a point within the areas ABCA and JKLMJ, particularly DEF and PQSR, of FIGS. 1 and 2, respectively.

While the subject invention has been primarily described in connection with the production of wire, particularly stranded wire for marine cable for use in stagnant seawater, steels contemplated herein are useful for a host of articles including the following: structural members, .screens, precision forgings, prosthetic devices, tubing, containers, etc.

As will be understood by those skilled in the art, the

term balance" or balance essentially used in referring to the iron content does not exclude the presence of other elements commonly present as incidental constituents, e.g., deoxidizing and cleansing elements (zirconium, boron, calcium, magnesium, and titanium) and impurities ordinarily associated therewith in amounts which do not affect the basic characteristics of the alloy. In this connection, a small but effective amount, e.g., 0.25 percent, up to 2 percent columbium can be employed, particularly in conjunction with carbon contents not exceeding 0.06 or 0.08 percent to improve corrosion resistance. Tantalum can be used in lieu of columbium, two parts of tantalum for one part of columbium. Columbium particularly is a rather strong ferrite promoter and if used, should be assigned a value of unity in computing the Ferrite Index. Up to 5 percent copper can be present. Constituents such as sulfur, phosphorus, hydrogen and oxygen should be maintained at low levels consistent with good commercial practice.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention,as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

1. A stainless steel consisting essentially of from 14 to 27.65 percent chromium, about 3 to 12 percent molybdenum, up to 17.5 percent nickel, cobalt present in an amount up to about 25.5 percent, the sum of the chromium plus twice the molybdenum being greater than 28.5, carbon up to 0.3 percent, up to 0.3 percent nitrogen, the sum of the carbon plus nitrogen not exceeding 0.3 percent, up to 2 percent silicon, up to 5 percent manganese and the balance essentially iron, the Ferrite Index being correlated with the Austenite Index so as to represent a point within the area ABCA of FIG. 1 and with the Austenite Stability Index so as to represent a point within the area JKLMJ of FIG. 2 of the accompanying drawing.

2. A stainless steel in accordance with claim 1 containing at least 1 percent nickel and 15 percent cobalt.

3. A stainless steel in accordance with claim 2 in which the sum of the chromium plus twice the molybdenum is at least 30.

4. A stainless steel in accordance with claim 2 containing from 0.01 to 0.08 percent carbon, the sum of the carbon plus nitrogen not exceeding 0.2 percent.

5. A stainless steel according to claim 2 in which the Ferrite Index and Austenite Index values are correlated to represent a point within the area DEFD of FIG. I.

6. A stainless steel according to claim 5 in which the area is DEI-IG of FIG. 1.

7. A stainless steel according to claim 2 in which the Ferrite Index and Austenite Stability Index values are correlated to represent a point within the area PQRS of FIG. 2.

8. A stainless steel in accordance with claim 2 in which the Ferrite Index is correlated with the Austenite Index so as to represent a point within the area DEFD of FIG. 1 and with the Austenite Stability Index so as to represent a point within the area PQRS of FIG. 2.

9. A stainless steel in accordance with claim 2 containing about 16 to 22 percent chromium, about 4 to 8.5 percent molybdenum, the chromium plus twice the molybdenum being at least about 30, about 2.5 to 6.5 percent nickel, about 15 to 25 percent cobalt, about 0.01 to 0.15 percent carbon, up to 0.0l percent nitrogen, the sum of the carbon plus nitrogen not exceeding 0.2 percent, up to about 0.75 percent silicon and up to about 2 percent manganese.

10. A stainless steel in accordance with claim 2 containing 17 to 20 percent chromium, 5.5 to 7.5 percent molybdenum, the sum of the chromium plus twice the molybdenum being at least 30.5, 3 to 5 percent nickel, 17 to 23 percent cobalt, 0.01 to 0.08 percent carbon, up to 0.04 percent nitrogen, up to 0.75 percent silicon and up to 2 percent manganese.

11. A stainless steel wire produced from a steel having a composition as set forth in claim 1.

12. A stainless steel wire produced from a steel having a composition as set forth in claim 3.

13. A stainless steel wire produced from a steel having a composition as set forth in claim l0.

14. A stainless steel wire produced from a steel having a composition as set forth in claim 9.

15. A work hardened article produced from a stainless steel having a composition in accordance with claim 1.

16. A work hardened article produced from a stainless steel having a composition in accordance with claim 3.

17. A work hardened article produced from a stainless steel having a composition in accordance with claim 10.

18. A work hardened article produced from a stainless steel having a composition in accordance with claim 9.

19. A stainless steel consisting essentially of from 14 to 27.65 percent chromium, about 3 to 12 percent molybdenum, being up to 17.5 percent nickel, cobalt present in an amount up to about 25.5 percent, the sum of the chromium plus twice the molybdenum being greater than 28.5, carbon up to 0.3 percent, up to 0.3 percent nitrogen, the sum of the carbon plus nitrogen not exceeding 0.3 percent, up to 2 percent silicon, up to 5 percent manganese, up to 2 percent columbium, up to 4 percent tantalum, the sum of the percentage of columbium plus one-half the percentage of tantalum being up to 2 percent, up to 5 percent copper, and the balance essentially iron, the Ferrite Index being correlated with the Austenite Index so as to represent a point within the area ABCA of FIG. 1 and with the Austenite Stability Index so as to represent a point within the area JKLMJ of FIG. 2 of the accompanying drawing.

20. A stainless steel in accordance with claim 19 containing at least 2 percent nickel, at least 15 percent cobalt, and in which the sum of the chromium plus twice the molybdenum is at least 30.

21. A stainless steel in accordance with claim 19 which contains at least 15 percent cobalt and a small but effective amount of columbium up to 2 percent to improve corrosion resistance.

22. A stainless steel in accordance with claim 19 containing at least 15 percent cobalt.

"H050 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,772,005 Dated November 1- 73 Inventors; JOHN JOSEPH deBARBAD'ILLO,II

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 1, line 56, for "This" read "That" Col. 4, line 56, for "where" read .""there" Col. 4, line 65, for "0.1 to 0.2" read "0.1 or 0.2" Col. 6, lir le 27, for "on" read "one" Col. 12, daine I, delete "being" Signed and sealed this 9th day of July 197 (SEAL) Attest:

MoCOY M.GIBSON,JR. "C. MARSHALL DANN Attesting Officer Commissioner" of Patents 

2. A stainless steel in accordance with claim 1 containing at least 1 percent nickel and 15 percent cobalt.
 3. A stainless steel in accordance with claim 2 in which the sum of the chromium plus twice the molybdenum is at least
 30. 4. A stainless steel in accordance with claim 2 containing from 0.01 to 0.08 percent carbon, the sum of the carbon plus nitrogen not exceeding 0.2 percent.
 5. A stainless steel according to claim 2 in which the Ferrite Index and Austenite Index values are correlated to represent a point within the area DEFD of FIG.
 1. 6. A stainless steel according to claim 5 in which the area is DEHG of FIG.
 1. 7. A stainless steel according to claim 2 in which the Ferrite Index and Austenite Stability Index values are correlated to represent a point within the area PQRS of FIG.
 2. 8. A stainless steel in accordance with claim 2 in which the Ferrite Index is correlated with the Austenite Index so as to represent a point within the area DEFD of FIG. 1 and with the Austenite Stability Index so as to represent a point within the area PQRS of FIG.
 2. 9. A stainless steel in accordance with claim 2 containing about 16 to 22 percent chromium, about 4 to 8.5 percent molybdenum, the chromium plus twice the molybdenum being at least about 30, about 2.5 to 6.5 percent nickel, about 15 to 25 percent cobalt, about 0.01 to 0.15 percent carbon, up to 0.01 percent nitrogen, the sum of the carbon plus nitrogen not exceeding 0.2 percent, up to about 0.75 percent silicon and up to about 2 percent manganese.
 10. A stainless steel in accordance with claim 2 containing 17 to 20 percent chromium, 5.5 to 7.5 percent molybdenum, the sum of the chromium plus twice the molybdenum being at least 30.5, 3 to 5 percent nickel, 17 to 23 percent cobalt, 0.01 to 0.08 percent carbon, up to 0.04 percent nitrogen, up to 0.75 percent silicon and up to 2 percent manganese.
 11. A stainless steel wire produced from a steel having a composition as set forth in claim
 1. 12. A stainless steel wire produced from a steel having a composition as set forth in claim
 3. 13. A stainless steel wire produced from a steel having a composition as set forth in claim
 10. 14. A stainless steel wire produced from a steel having a composition as set forth in claim
 9. 15. A work hardened article produced from a stainless steel having a composition in accordance with claim
 1. 16. A work hardened article produced from a stainless steel having a composition in accordance with claim
 3. 17. A work hardened article produced from a stainless steel having a composition in accordance with claim
 10. 18. A work hardened article produced from a stainless steel having a composition in accordance with claim
 9. 19. A stainless steel consisting essentially of from 14 to 27.65 percent chromium, about 3 to 12 percent molybdenum, being up to 17.5 percent nickel, cobalt present in an amount up to about 25.5 percent, the sum of the chromium plus twice the molybdenum being greater than 28.5, carbon up to 0.3 percent, up to 0.3 percent nitrogen, the sum of the carbon plus nitrogen not exceeding 0.3 percent, up to 2 percent silicon, up to 5 percent manganese, up to 2 percent columbium, up to 4 percent tantalum, the sum of the percentage of columbium plus one-half the percentage of tantalum being up to 2 percent, up to 5 percent copper, and the balance essentially iron, the Ferrite Index being correlated with the Austenite Index so as to represent a point within the area ABCA of FIG. 1 and with the Austenite Stability Index so as to represent a point within the area JKLMJ of FIG. 2 of the accompanying drawing.
 20. A stainless steel in accordance with claim 19 containing at least 2 percent nickel, at least 15 percent cobalt, and in which the sum of the chromium plus twice the molybdenum is at least
 30. 21. A stainless steel in accordance with claim 19 which contains at least 15 percent cobalt and a small but effective amount of columbium up to 2 percent to improve corrosion resistance.
 22. A stainless steel in accordance with claim 19 containing at least 15 percent cobalt. 